Manufacture and separation of thiourea from ammonium thiocyanate



M. DONAUER March 6, 1934.

MANUFACTURE AND SEPARATION OF THIQUREA FROM AMMONIUM THIOCYANATE FiledJan. 3l, 1931 INVENTOR. Wax ONQc/e/ TTZ' NEY.

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MN N rib- Patented Mar. 6, 1934 MANUFACTURE AND THIOUREA F R -M CYAN ATESEPARATION 0F AMIMONIUM THIO- Max Donauer, Arlington, J., assiznor toThe Koppers Company of Delaware, a corporation of Delaware ApplicationJanuary 31,

14 Claims.

My invention relates to the manufacture'and purification cof chemicalcompounds, and especially to the conversion of ammonium thiocyanate tothiourea and the product obtained thereby. In a more specific aspect, myinvention relates to the sepaartin of chemical compounds, such asammonium thiocyanate and thiourea.

An object of my invention is to provide a process of separating chemicalcompounds such. as ammonium thiocyanate and thiourea. A

A second object oi my invention is to provide a process whereby ammoniumthiocyanate is more eiliciently and economically converted to thioureathan has been possible in the past.

Another object of my invention is to provide a process oi manufacturingthiourea wherein the cooling effect obtainable by dissolving ammoniumthiocyanate is utilized to facilitate the recovery oi the thiourea.

My invention has for further objects such other advantages and resultsas are found to obtain in the process and product hereinafter describedand claimed.

It is well known that ammonium thiocyanate The extent of vthisconversion depends on the temperature to which the thiocyanate isheated, the duration of the period throughout which the conversion massis maintained at that temperature, and other factors. At 140 C., forexample, equilibrium is reached when the conversion mess contains 28%thiourea, but a period of eight hours is required to reach this point.At 180 C., the equilibrium is at 21% thiourea, but this is reached uione-half hour.

it 180, however, the loss by decomposition of thiourea into hydrogensulphide and cyanf amide is comparatively high. Furthermore, thesedecomposition products lead to further losses ii allowed to remain inthe conversion mixture, so that the total loss may amount to iron13%to5% or the total ammonium thiocyanate, or to ci the recoverablethioureaat each heating. There is very little loss at 140.

The reaction which liberates moet oi the HBS during conversionV ispresumably:

i The cyanamide reacts with ammonium thiocya- 1931, serial No. 512,658

nate to form guanidine thiocyanate, which remains as an impurity in theconversion mass:

CN.NH2 +NH4CNS= CNS.CN3H6 guanidine thiocyanate.

1f the His is not removed, it reacts with NHlcNs to form ammoniumtrithiocarbonatel:

which causes further losses by sbliming as such, or by breaking down toform carbon bisulphide and ammonium sulphide.

It is obviously important to keep losses by decomposition as low aspossible because, in order to obtain an amount of thiourea equivalent tothe amount oi thiocyanate started with, it is necessary to re-processthe unconverted residues several times. For instance, if the recoverableyield oi thiourea from one conversion is 15% to 18% of the conversionmass, by weight, which is a typical range for processes known to theprior art, it is necessary to ire-process the unconverted residues sixtimes or more in order to obtain complete conversion to thiourea. Aslosses by decomposition are multiplied by re-processing, it hasfrequently happened in the past that only to 60% oi the thiocyanate-hasbeen recovered as thiourea. t/

In the processes .irnown to the prior art, 'arnm nium thiocyanate wasusually heated to a temperature between 140 and 180 C. and held at thattemperature for a sumcient period to allow the conversion to come toequilibrium. The confa version mass was then dissolved in water,preterably while still molten, and the solution was transferred to ajaclreted Irettleor other suitable cooling vessel, cooled, as with icewater, to about 25 C., and then with reirigerating brine to about il" C.

Below 25 a double salt oi thiourea and ammonium thiocyanate began tocrystallize out in long fine needles. As the cooling and crystallizationproceeded, these needle-like crystals caused the slurry to thicken untilit almost solidified, malsing it very dimcult to handle, necessitatingcon- 10@ stant stirring, and greatly decreasing the em;- ciency ofcooling. When crystallization was com pleted, the double salt was lteredod and re-dis'- solved, and cooled again to crystallize thiour'ea.

It a very pure product was desired, this thiourea 105 :from theconversion'mass. In the process oimy lio invention the ammoniumthiocyanate is quickly heated to a temperature somewhat above the byholding the temperature ol' a molten batch at" about 160 for about twohours, and then letting it drop to about 110 over night, for example.

The molten. mixture is then discharged onto a cooled granulating disc ora'iiaking roll or the like, on which cooling and solidication arecominto flakes or granular particles.

This is a very eicient means of cooling, and it allows any H2S liberatedduring conversion to escape freely, so that the liquor obtained when themass is redissolved is substantially free from decomposition productsand consequently more suitable for re-processing. It also leaves themass in such a condition that the cooling effect obtained by addingwater to solid ammonium thiocyanate, which dissolves endothermically,can be utilized to advantage in separating the thiourea from themixture.

The dry flaked or granulated conversion mass is then transferred to adissolving tank and mixed with the proper amount of aqueous liquid suchas water and/ or mother liquor from thiourea recrystallization. Thisliquid ispreferably at a temperature of about 10 to 14 C., and ispreferably supplied in such quantity that the resulting solution has anammonium thiocyanate content of about 535 grams per liter.

When water at this 'temperature is added to the conversion mass,e theammonium thiocyanate contained therein goes into solution rapidly andthe temperature of the solution drops to about -10 C. With the water orliquor pre-cooled to 4 C., temperatures as low as 20 C. havebeenobtained by dissolving the conversion mass.

leus At 10 C., thiourea has a solubility in water of only 35 grams perliter, while ammonium thiocyanate is still soluble up to 535 grams perliter. 'This results in a solubility ratio of more than 15 to 1, whichis exceptionally high and better than is obtainable with most of themore expensive organic solvents.

Since this satisfactory ratio is obtained by starting the solution withthe solvent water or mother liquorat a temperature of 10 to 14 C., asstated hereinabove, it is seldom desirable to incur extra expense byprecooling the solvent to lower temperatures. Water at this temperatures available in many localities, for most of the year at least, withoutany artificial cooling,

vand water occurring I,naturally at somewhat higher temperatures, suchas 22 C., has also given satisfactory results at this stage of theprocess. Ammonium thiocyanate dissolves rapidly from the thin iiakesorgranules of the conversion mass, but the double salt of ammoniumthiocyanate and thiourea does not dissolve appreciably and remains as aflne, white, powdery precipitate. A slurry is formed which is generallyquite limpid, and much more easily filtered and otherwise handled thanthe slurry formed in previous processes, when the needle-like crystalsof double salt would nearly cause the slurry to solidify.

'I'he double salt is filtered out as before, 'and re-dis'solved in hotwater to which decolorlzing carbon and a filter aid may be added ifdesired. This solution is filtered while hot to remove `the carbonand/or other solids, and is then cooled to recover the thiourea incrystalline form. If desired, a further re-crystallization may bepracticed, as before.

The mother liquor from which the double salt is separated isconcentrated and evaporated to dryness, and the mother liquor solidsthus obtained are added to fresh ammonium thiocyanate and re-processed.The. mother liquor from the irst re-'crystallization of thiourea lmay beused fordissolving. the ilaked conversion mass, and the mother liquorfrom the final re-crystallization, when practiced, is suitably used forthe same purpose or for the ilrst recrystallization. pleted, and theconversion mass is flnely divided By means of these improvements on theprocesses of the prior art, I have obtained yields of thioureaamountingv to and over, as compared withV 50% or thereabouts reported inthe literature, and I have also effected a considerable saving in thecost of processing.

With the objects set forth above andother objects in view, I nowdescribe with reference to the accompanying drawing a preferred methodof practicing my improved process of thiourea manufacture and recovery.In the. drawing,

The single figure is a partially diagrammatic view partly in elevationand partly in vertical section of apparatus suitable forlthe practice ofmy invention.

Crystalline gam'monium thiocyanate and/or mother liquor. solids aremelted in a jacketed melting pot 1 or other suitable device. Thismelting is preferably conducted under vacuum maintained by means of avacuum pump (not shown) connected with the pot by means of a pipe 2.Heat is supplied by steam admitted to the jacket of the pot through apipe 3. Spent steam and/or condensate are discharged through a pipe 4. iI

. When the desired temperature is reached-for example, 160 C.-the moltenconversion mass is dischargedthrough a pipe 6 into a holding tank 12o 'Iby air admitted through a pipe 8 or by ,other suitable means ofldelivery. The tank 7 is insulated or provided with a jacket by means ofwhich the desired temperature within the tank can be maintained. In thepresent instance, a jacketed vessel is used. Steam is supplied to thejacket through a pipe 9 and discharged through a pipe 10. In someinstances it may be desirable -to eliminate the melting pot 1 andconduct the melting in the holding tank 7.

After the molten conversion mass has re-- mained in the tank 'I untilthe desired conversion has been obtained and the temperature has fallennearly to the solidifying point, the mass is withdrawn through a pipe 12onto a aking roll 13 or other suitable granulating device, such asrapidly rotating air-cooled disk. 9

The roll 13 is revolved slowly by a motor 14 and a belt 15, or othersuitable driving mechanism, and is maintained at a temperature lowenough to completely solidify the conversion mass by cooling watersupplied preferably to the interior Aof the roll through a pipe 16. Themolten conversion mass is distributed in a thin layer onthe surface ofthe roll. Its heat is transferred through the roll to the cooling water,and the mass is solidifled in a thin sheet. This sheet is then removedfrom the roll by a scraper 17 in the form of thin flakes, which aretransferred bya conveyor 18 or other suitable means into a hopper 20.

The-naked conversion mass is discharged from the hopper 20 into adissolving tank 21 wherein it 'is mixed with a suitable quantity o!water and/or mother liquor which is delivered preferably at atemperature o1' 10 to 14 C. from a. tank 23 through a pipe 24 into thedissolving tank. During this mixing with cold water the ammoniumthiocyanate is rapidly dissolved, cooling the solution to about 10 C.,as stated hereinabove. The double salt of ammonium thiocyanate andthiourea, being substantially insoluble at this temperature, remains.undissolved in the form of a white precipitate, which-forms a limpidslurry with the ammonium thiocyanate solution. This slurry is deliveredby pump 25 through a lter 26 which separatesV the double salt from themother liquor. The mother liquor, consisting principally of ammoniumthiocyanate solution, is discharged through a pipe 27 into an evaporator23 and the double salt is discharged into a hopper 29 or othersuitablefeeding device which delivers it into a dissolving tank 31.

In tank 31 the double salt is dissolved in water supplied through a pipe33 and/or mother liquor from a later stage of the process suppliedthrough a pipe 34. This water or liquor is heated to a temperature atwhich the double salt is comparatively soluble, such as about 60 C., bymeans of a steam coil 35 in the tank, and a stirring device36 is used toassist in dissolving the double salt if necessary.

Decolorizing carbonand/or a filter aid may be added to the solution ifdesired to assist in the removal of organic coloring matter and otherimpurities such as carbon and sulphur liberated by decompositionoccurring during conversion. The

hot solution is forced by compressed air admitted through a pipe 37, orother suitable delivery means, through a` pipe 38, a iilter 39 wherebysolids are removed, and a pipe 40, into a storage tank or cooling tank.42.

- In this tank ll2 the solution is cooled by a cooling coil 43 or othersuitablemeans to about 0 C. to crystallize the thiourea. The slurry isthen blown by air supplied through a pipe 44, or otherwise transferred,through a pipe 46 to a filter 48 by which the crystallized thiourea isretained. 'These thiourea crystals usually contain 95% or` morethiourea, and are sumciently pure for most purposes.

The mother liquor passes from the lter 48 through pipes 49 vand 50 totank 23, from which it is withdrawn and used in dissolving a furtherquantity of the ilaked conversion mass. .The mother liquor can be usedalone for this purpose, or after mixing with water supplied to the tank23 through a pipe 52. As stated hereinabove, the water or liquor or bothare preferably at temperatures between 10 and 14 C. when used, and if itis necessary to cool them to this temperature, such cooling can beeiected by any desired means, such as by use of a cooling coil in thetank 23, adding ice to the liquor, etc.

. If further purication of the thiourearrecov ered in the filter 48 isdesired, the crystals are re-dissolved, preferably in fresh water, in 'atank 54 which may be jacketed or provided with a coil 55 or othersuitable means of heating and cooling. The thiourea crystals areadmitted to the vessel 541mm a hopper 56 or other suitable feedingdevice, and mixed, as by a. stirrer 58, with water admitted through apipe 59. The mixture is heated until the crystals arecompletelydissolved. The solution is then cooled again to crystallizethiourea, `and the slurry is transferred through a pipe 60 -to a lter 62by air admitted to vessel 54 through a pipe 63.

The thiourea crystals recovered by the lter. 62 are very pure, having athiourea content o1' 99% or more when dry. The final mother liquor isdischarged fromv the lter through a pipe 65, from which it may ilowthrough pipes 66 and 50 to tank 23 or through pipe 34 to tank 31. lineither case it is reused in an earlier stage of the process.

The mother liquor separated rorn'the double salt by the filter Z'andtherefroml transferred to the evaporator 28 `contains about 535 gramsper liter of ammonium thiocyanate. This liquor is concentratedsubstantially to dryness in the evaporator 28, preferably under vacuum,or -it may be partially dehydrated in this evaporator 28 and thentransferred as by a pump 68 through pipes 69 and 70 into the melting potl, in which evaporation is completed and the mother liquor solids aremelted.

A vacuum is maintained in the evaporator 28 by means of a vacuum pump(not shown) connected to the vapor outlet 72. The evaporator is heatedby steam which is supplied through a pipe 73 and. discharged through .apipe 74, or by other suitable means.

Fresh ammonium thiocyanate crystals are added to the mother liquorsolidsin the vessel l, and the mixture is melted 'and heated to theconversiontemperature, and reprccessed as described hereinabove.

By means of my present invention, considerim@ able improvement is madein the eclency and economy of converting ammonium thiocyanate tothiourea. H25 formed by decomposition of the ammonium thiocyanate isallowed to escape readily, thus lessening the lossby decomposition andlll improving the quality of Vthe mother liquor solids returned forre-processing. The endothermic solution of ammonium thiocyanate isutilized to advantage, and results in a considerable reduction in thecooling costs. Y

The double salt of thiourea is obtained in a more easily handled andreadily lterable form thanmin the previously known methods, and theamount ofcrystallized thiourea recovered from one conversion isincreased to about 22% or 125 higher, thus decreasing the amount ofre-processing necessary to obtain an amount of thiourea equivalent tothe initial amount oithiocyanate.

However, although my invention has been described with reference to themanufacture and la@ purification of thiourea, it is not limited to suchuse. It is also adapted to use in other chemical processes in which itis desired to separate two or more compounds, especially when thedissolving of the more soluble compound is an' endothermic process.Processes in which this principle may be employed to advantage includethe recovery of ammonium thiocyanate prepared by heating carbondisulphide and ammonium carbonate under pressure, and the recovery ofpotassium thiocyanate made by heating potassium ldd 'ferrocyanide andsulphur, for example.

It will be obvious' to those skilled in the art that variousmodifications can be made in the several parts of my apparatus and inthe several'stepsofmy process without departing from ythe spirit of myinvention,y and it is my intention to cover in theclaims suchmodications as are included within the scope. thereof.

I claim as my invention: 1. In a process of separating a chemicalcompound from a second compound which dissolves .endothermically thestep which comprises contacting a mixture while at ordinary roomtemperature and containing the two compounds witha quantity ofsolventwhile at ordinary room tem perature and which'dissolves the saidsecond compound and is thereby cooled to a temperature at) which -theiirst compound is relatively insolu le. v

2. In a process 'of separating a chemicalv compound from another moresoluble compound, the solution oi' which is an endothermlc process, thestep which comprises contacting a mixture while at ordinary roomtemperature and containing the two compounds with a quantity o! solventwhile at ordinary room temperature and which dissolves the more solublecompound and is thereby -cooled to a temperature at which the othercom-A pound is relatively insoluble. l

3. In a process of separating a chemical compound from a mixtureincluding a 'second compound which dissolves endothermically, the stepswhich comprise adding said mixture at ordinary room temperature to waterat ordinary roomtem perature and dissolving the said -second compoundtherein to iorm a solution which in forming cools itseli' to atemperature at which the tirs compound is substantially insoluble.

4, The process of separating a compound `from a second more solublecompound which goes into solution endothermically. which comprisesdividing a mixture containing the two compounds into small particles,contacting ,the particleswhile at a temperature not substantially aboveordinary room temperature with a solvent whileat a temperature notsubstantially above ordinary room temperature in which the more solublecompound dissolves to form a solution which is thereby cooled to atemperature at which the rst compound is relatively insoluble, andseparating the solution from 'the undissolved compound.

5. The process of separating a compound from a second compound whichvdissolves endothermically in water, which comprises contacting a mixturecontaining the two compounds while at aV temperature not substantiallyabovel ordinary room temperature with a quantity ofwater while at atemperature not substantially above ordinary room temperature and which'dissolves the said second compound and is thereby cooled to atemperature at which the ilrst compound is relatively insoluble, andseparating the solution from the undissolved compound.

8. In a process of separating' thiourea from ammoniuml thiocyanate thestep which comprises contacting a mixture while at a temperature notsubstantially above ordinary room temperature and containing the twocompounds with a quantity of water while at a ltemperature notsubsantially above ordinary room temperature and which holds thesolution from reaching saturation, enough ammonium thiocyanate beingpresent that byits solution the reaction mixture is thereby cooled to atemperature at which thiourea is relatively insoluble.

7. In a process oi' separating thiourea from l ammonium thiocyanate, the`step which comprises contacting a mixture while at a temperature notsubstantially above ordinary room temperature and containing the twocompounds with a quantity oi water while at a temperature not sub-1,940,7ss y l tion, enough of the ammonium thiocyanate being presentthat by its solution the reaction mixture i'orms a solution containingabout 535 grams per liter and is thereby cooled to a temperature atwhich the thiourea is relatively insoluble.

8. In a process `of separating4 thiourea from ammonium thiocyanate, thestep which comprises contacting a mixture while at a temperature notsubstantially' above ordinary. room' temperature and containing the saidcompounds with a quan-` tity of cold water while` at atemperature notsubstantially above ordinary 'roomv temperature and which holds thesolution from reaching supersaturation, enoughof the ammoniumthiocyanate being present that by its solution the reaction mixture isthereby cooled to a point at which the thioureau is precipitated.

9. In a process of separating` thiourea from ammonium thiocyanate, thestep which comprises contacting particles of a mixture while at atemperature not substantially above ordinary rocm temperature andcontaining the said compounds with a quantity of water at a temperaturebelow 25 C. which holds the solution from reaching saturation, enough ofthe ammonium thiocyanate being present that by its Msolution thereaction mixture is thereby cooled to a point at which the thiourea isprecipitated.l 10. In a process of separating thiourea from ammoniumthiocyanate, the step which comprises contacting particles of a mixtureWhile at a temperature not substantially above ordinary room temperatureand containing the said compounds small particles, contacting theparticles while at 1m substantially ordinary room temperature with aquantity of water while at substantially ordinaryroom temperature andwhich holds the solution from reaching supersaturation, enough ammoniumthiocyanate being present that by its solution the` reaction mixture isVthereby cooled to a temperature at which the thiourea is relativelyinsoluble, and separating the solution from the undissolved thiourea.

12. The process of separating thioureaffrom ammonium thiocyanate whichcomprises -divid-y ing a mixture containing the said vcompounds intosmall particles, contacting the particles while at substantiallyordinary room'temperature with a quantity lof cold water which sufilcestn hold the solution from reaching saturation, enough of the ammoniumthiocyanate being present that by its solution the reaction'mixtureforms a solution containingmore than 500 grams of NHiCNS per liter andis thereby cooled to a temperature at which lthiourea is precipitated,`and filtering precipitated `thiourea from the solution.

13. The process `oi manufacturing thiourea which comprises meltingammonium thiocyanate and heating the molten mass`to a temperature aboveC. to form thiourea, lcooling the partially converted mass slowly to atemperature near the melting point, solidifying and further' cooling themixture of ammonium Vthiocyanate -and thiourea, contacting the solidiedmixture while at a temperature not substantially above ordinary roomtemperature with a quantity o! cold water which holds the solution fromreach- 'ing supersaturation, enough ammonium'thioonto a relatively coolsurface and thereby solidifyins it, removing the solidified mixture fromthe cool suri'ace in the fom o! small particles, contaotinz thepaxtieleswhile ata temperature not substantially above ordinary 'room temperaturewith a quantity of water at a temperature below 25 C. which holds thesolution from saturation. enough of the ammonium vtixiooyanaie beingpresent that by its solution the reaction mixture is thereby cooled to atemperature at which thiourea is relatively insoluble, separating thesolution from the undissoived solids. dissolving the solids in water ata'hlsher temperature, and cooling the solution thereby obtained torecrystalliae thiourea.

MAXDONAUIR;

